1. Field of the Invention
The present invention relates to an electrical installation having a plurality of interconnected batteries each having multiple cells, and more particularly to balancing the state of charge of all the cells in those batteries.
2. Description of the Related Art
Lithium-ion batteries are commonly used to power electronic devices and electrically operated vehicles. This family of rechargeable batteries provides relatively high, energy density and relatively slow loss of charge when not in use. As with other types of batteries, lithium-ion devices comprise a plurality of cells connected in series to produce the desired output voltage for the battery. Over its useful life, the battery may be charged and discharged for hundreds or even thousands of cycles. As this occurs, the individual cells age differently. As a consequence, some cells of the battery may become mismatched in terms of voltage level and state of charge with respect to other cells. If this mismatch is not corrected, one or more of the cells can become overcharged or undercharged, either of which can lead to failure of the battery.
To counteract the effects of cell mismatch, various balancing techniques have been devised for forcing all the cells to have identical voltage. This is accomplished by a balancing circuit that draws energy from cells having a higher voltage and applies that energy to a shared bus. Other cells having a lower voltage are charged further using energy from the shared bus. Each cell is associated with a bidirectional converter that controls the flow of energy between the cell and the shared bus. A balancing technique that is based on individual cell voltages does not ensure all the cells will have same state of charge, as the voltage does not directly indicate the state of charge where the cells differ in capacity and internal resistance.
Although such prior techniques approximately balanced the cells within a battery, those techniques did not account for cell mismatch between multiple batteries connected in series. Several batteries are often connected in series in equipment requiring a relatively high voltage. In this type of installation, however, the cells in one battery may be balanced to a level that is mismatched to the level at which the cells in an adjacent battery are balanced. In that installation, the capacity of the “worst” battery determines the capacity of the series. Therefore, it is desirable to be able to balance all the cells in the series of interconnected batteries to the same level so that the average capacity of the individual cells determines the total capacity of the system.
Nevertheless, the batteries in many applications must be physically separated from one another by a significant distance. For example, in an auxiliary power supply for a yacht, the relatively large and heavy batteries are distributed through the vessel to balance the weight for proper buoyancy. Because it is also desirable to maximize the living space within the yacht, the individual batteries must be placed at different locations where storage space is available. This physical separation of the different batteries presents an additional challenge in providing a mechanism to balance all the batteries to a common level.